Counterforce mechanism and methods of operation thereof

Abstract

A counterforce mechanism is arranged to exert a force on an object to maintain the object at a desired position. The mechanism comprises a driven body, a drive for moving the driven body, and a resilient coupling arrangement for coupling the driven body to a portion of an object. A control arrangement is arranged to output a drive signal to the driven body drive to move the driven body to a location where it exerts a force on the object via the coupling arrangement, such that the force counteracts an opposing force acting on the object and the mechanism holds the portion of the object at the desired position. A machine axis and a machine tool incorporating such a counterforce mechanism are also described, together with methods of operation thereof.

Claims

1. A machine axis for a machine tool, comprising: a support for supporting a tool or a workpiece; a support drive for moving the support; and a counterforce mechanism comprising: a driven body; a drive for moving the driven body; a resilient coupling arrangement; a control arrangement, wherein the coupling arrangement couples the support and the driven body to each other, and the control arrangement is arranged to output a drive signal to the driven body drive to move the driven body to a location where it exerts a force on the support via the coupling arrangement, such that the force counteracts an opposing force acting on the support and the counterforce mechanism holds the support at the desired position, and the driven body is moveable along at least one guideway which guides the driven body in the direction of a linear reference axis and prevents rotation of the driven body relative to the linear reference axis of the guideway; and a safety mechanism for resisting a decrease in the spacing between the support and the driven body beyond a minimum threshold or for resisting an increase in the spacing between the support and the driven body beyond a maximum threshold.

2. A machine axis of claim 1, wherein the counterforce mechanism is configurable to exert a force on the support such that the support is held at a predetermined height above a support surface.

3. A machine axis of claim 1, wherein the axis of motion of the support is linear.

4. A machine axis of claim 3, wherein the support drive is a linear motor direct drive.

5. A machine axis of claim 1, wherein the support and driven body are moveable along a common guideway.

6. A machine axis of claim 1, including a sensor for detecting displacement of the support from the desired position and outputting a signal to the control arrangement in response thereto.

7. A machine axis of claim 1 including: a damping mechanism for decelerating one of the support and the driven body relative to the other.

8. A machine axis of claim 1, wherein the driven body is moveable relative to the same axis of motion as the support.

9. A machine axis of claim 1, wherein the driven body is located within the support.

10. A machine axis of claim 1, wherein the control arrangement is configured to adjust the position of the driven body having regard to the power demand of the support drive.

11. A machine axis of claim 1, wherein the driven body drive is able to hold the driven body in position without requiring electrical power.

12. A machine tool including a machine axis of claim 1.

13. A machine axis of claim 1, including a brake mechanism for resisting movement of the support when the driving force exerted by the support drive is removed.

14. A machine axis for a machine tool, comprising: a support for supporting a tool or a workpiece; a support drive for moving the support; and a counterforce mechanism comprising: a driven body; a drive for moving the driven body; a resilient coupling arrangement; a control arrangement, wherein the coupling arrangement couples the support and the driven body to each other, and the control arrangement is arranged to output a drive signal to the driven body drive to move the driven body to a location where it exerts a force on the support via the coupling arrangement, such that the force counteracts an opposing force acting on the support and the counterforce mechanism holds the support at a desired position, and wherein an axis of motion of the support is rotary; and a safety mechanism for resisting a decrease in the spacing between the support and the driven body beyond a minimum threshold or for resisting an increase in the spacing between the support and the driven body beyond a maximum threshold.

15. A machine axis of claim 14, wherein the support drive is a rotary motor direct drive.

16. A machine axis of claim 14, wherein the support is rotatable in use about an axis, and the driven body is rotatable about an axis to an angular position dependent on the angular position of the support so as to exert a force on the support via the coupling arrangement to counteract torsional forces acting on the support.

17. A machine axis of claim 16, wherein the support and driven body are rotatable around a common axis of rotation.

18. A machine axis of claim 14, including a sensor for detecting displacement of the support from the desired position and outputting a signal to the control arrangement in response thereto.

19. A machine axis of claim 14, including: a damping mechanism for decelerating one of the support and the driven body relative to the other.

20. A machine axis of claim 19, including a brake mechanism for resisting movement of the support when the driving force exerted by the support drive is removed.

21. A machine axis of claim 14, wherein the driven body is moveable relative to the same axis of motion as the support.

22. A machine axis of claim 14, wherein the driven body is located within the support.

23. A machine axis of claim 14, wherein the control arrangement is configured to adjust the position of the driven body having regard to the power demand of the support drive.

24. A machine axis of claim 14, wherein the driven body drive is able to hold the driven body in position without requiring electrical power.

25. A machine tool including a machine axis of claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:

(2) FIG. 1 is a front perspective view of a vertical axis for a machine tool which embodies the present invention;

(3) FIG. 2 is an enlarged perspective view of the driven body forming part of the vertical axis shown in FIG. 1;

(4) FIGS. 3 and 4 are front perspective views of the machine axis of FIG. 1 with the support and the driven body alone mounted on the axis, respectively;

(5) FIG. 5 is an enlarged perspective view of the upper portion of a machine axis according to a further embodiment of the invention;

(6) FIG. 6 is an enlarged partially cut-away side view of the upper portion of a machine axis according to another embodiment of the invention;

(7) FIG. 7 is a front perspective view of a horizontal axis for a machine tool in accordance with the present invention;

(8) FIGS. 8 and 9 are front perspective and cross-sectional views, respectively, of a rotary axis for a machine tool according to a further embodiment of the invention;

(9) FIGS. 10 and 11 are a front perspective and transverse cross-sectional views, respectively of the resilient coupling arrangement of the machine axis depicted in FIGS. 8 and 9;

(10) FIG. 12 is a side perspective view of a machine base support unit according to another embodiment of the invention;

(11) FIG. 13 is a side perspective view of a machine tool support on four units of the form shown in FIG. 12; and

(12) FIG. 14 is a side perspective view of a machine tool supported by two units of the form shown in FIG. 12 in combination with four primary support springs.

DETAILED DESCRIPTION OF THE INVENTION

(13) The same reference signs are generally used in the drawings to refer to corresponding or similar features in modified and different embodiments.

(14) In the vertical axis 2 shown in FIGS. 1 to 4, a support in the form of a primary carriage 4 is moveable along a pair of guideways or guide rails 6 carried by a support column 1. A mount 8 is provided on the front face of the carriage for attaching a tool or workpiece. The primary carriage is driven along the guideways by a linear motor 10.

(15) A driven body or secondary carriage 12 is provided for movement along a direction parallel to the guideways of the primary axis. It moves along respective linear guideways 14 provided on a front vertical face of the support column 1. The secondary carriage is driven by a screw drive.

(16) The screw drive for the secondary carriage comprises a vertical screw 16 which is supported by a support block 18 and driven by a servo motor 20. A direct coupled angular encoder 22 is fitted to the servo motor 20 to provide feedback to a machine controller regarding the position of the secondary carriage. In the illustrated embodiment, the servo drive is orientated perpendicularly with respect to the screw axis, but it will be appreciated that other configurations may be adopted.

(17) Screw 16 engages a screw nut 24 mounted on the secondary carriage 12. Thus, rotation of the screw by servo motor 20 displaces the secondary carriage in the vertical direction.

(18) For the purposes of illustration, support column 1 is shown in FIGS. 3 and 4 with only one of the primary and secondary carriages and its associated drives and guideways, respectively.

(19) As shown in FIGS. 1 and 2, a resilient component in the form of a coil spring 26 is located with its axially opposed ends in engagement with the primary and secondary carriages, respectively. This provides a resilient coupling arrangement between the two carriages, and the secondary carriage is able to exert an upwards force on the primary carriage via this coupling arrangement to counteract gravitational forces acting on the primary carriage. The force from the resilient component is preferably applied to the primary carriage through the centre of gravity of the primary carriage mass. The lower end of the coil spring is supported by and attached to a mount 28 on the secondary carriage.

(20) A damping mechanism 30 is provided between the primary and secondary carriages for reducing the impulse exerted on each carriage as they approach each other. It is supported at its lower end by a mount 32 on the secondary carriage.

(21) In operation of the vertical axis shown in FIGS. 1 to 4, the primary carriage 4 is driven to a desired position with relatively high accuracy by the linear motor 10, under the control of a controller included in the machine tool of which the axis forms a part (such as a CNC control system). The secondary carriage 12 is simultaneously driven to a position below and adjacent to the primary carriage by a relatively low cost ball screw drive under the control of the controller with a relatively low, but adequate positioning accuracy. Coil spring 26 maintains engagement with the underside of the primary carriage 4, thereby applying a vertical axial force to the primary vertical axis carriage.

(22) The spacing between the primary and secondary carriages needed to exert a counterbalancing force equal and opposite to the gravitational forces acting on the primary carriage (and any components mounted on it) may be determined by turning off the drive for the primary carriage and allowing its weight to be borne solely by the secondary carriage. This will indicate the desired spacing, which can then be recorded in the machine controller for reference during a subsequent machining procedure. This procedure could be used to reset the magnitude of the correct spacing when the total mass of the primary drive and anything carried by it changes.

(23) The resilient coupling provided by the coil spring is arranged to have a relatively low spring rate (or equivalent characteristic in other forms of resilient component), such that variations in the force it applies on the primary carriage resulting from any errors in positioning of the secondary carriage are acceptably low.

(24) For example, the potential counterbalance force errors may be estimated by assuming that the secondary carriage position would be no further than 0.1 mm away from its demand, so the force applied to the primary carriage by the spring should be substantially constant over this variation. By way of illustration, a die spring of 50 mm outside diameter with a free length of 150 mm would compress by approximately 20 mm when supporting a 500 kg carriage. Therefore, a + or −0.1 mm potential positioning error for the secondary carriage gives a counterbalance force variation of as little as 25N (+ or −0.5%). For higher mass primary carriages, multiple springs (or other couplings) could be used to provide the required force.

(25) The machine axis receives control signals from a controller of the machine tool. In order to effect a movement of a tool or workpiece mounted on the axis, the controller sends signals to the drives for the primary and secondary carriages instructing each of them to carry out the same movement. Thus, the secondary carriage effectively mimics the movements made by the primary carriage so as to continously provide a counterbalancing force.

(26) If appropriate, a greater degree of control over the magnitude of the counterbalancing force could be obtained by providing a measurement device to monitor the spacing between the primary and secondary carriages. The machine controller could be arranged to adjust the position of the secondary carriage if necessary having regard to an output signal from the measurement device, in order to maintain a constant spacing and therefore a constant counterbalancing force. Alternatively, the position of the secondary carriage could be adjusted so as to minimise the current demand of the primary drive as measured by the machine controller.

(27) It will be appreciated that several modifications or variations to the configuration shown by way of example in FIGS. 1 to 4 are encompassed by the present invention.

(28) In the arrangement illustrated in FIGS. 1 to 4, the secondary carriage is positioned below the primary carriage, with the load support being provided by a compression spring 26. Alternatively, the secondary carriage could be positioned above the primary carriage, with the load support being provided by a tension spring (or other resilient component in tension). Such an arrangement is depicted in FIG. 5, in which the primary carriage is attached to the lower end of tension spring 26′. The secondary carriage is hung from, rather than supported by, the screw drive.

(29) In other configurations, this secondary carriage may be positioned to the side, behind or in front of the primary carriage. It could also be at least partially accommodated within the dimensions of the primary carriage. In the embodiment of FIG. 6, the secondary carriage 12 is located within the body of the primary carriage 4, to provide a more compact configuration. Coil spring 26 bears against an inner wall of the body.

(30) In other arrangements, the secondary carriage and its guideways 14 could be provided on the back or inside support column 1, and apply the preload force to the carriage via cables or chains running over pulleys, with the secondary carriage travelling in an opposite sense to the primary carriage. The coupling arrangement between the primary and secondary carriages would in this arrangement comprise cables or chains connected in series with a resilient component such as coil spring 26.

(31) The screw 16 of the screw drive associated with the secondary carriage could be of a plain thread type, or a rolling element type such as a ball screw or roll screw. The degree of smoothness of motion or positioning accuracy of the second drive is low, as these effects will be substantially isolated from the primary carriage by the coupling arrangement.

(32) The screw 16 may have a relatively fine pitch (that is a low helix angle) such that the drive will be non-reversible by its payload (that is the primary and secondary carriages). Alternatively, the screw could have a coarser pitch with a high ratio or worm/wormwheel gear box incorporated into the drive system, such that the drive system is again non-reversible. In this way, the secondary carriage and therefore the primary carriage would hold their position with the power removed from their respective drives.

(33) In another configuration, a brake could be provided to prevent rotation of the screw 16 or its servo motor 20 when a stop condition is required.

(34) A range of other types of drive may form the secondary drive which could satisfy the relatively low positioning accuracy requirement for the secondary carriage drive. For example, it may be in the form of a rack and pinion drive, a friction drive, a capstan drive, or pneumatic or hydraulic cylinders with positioning capability.

(35) The secondary carriage may be utilised to carry out additional functions. For example, connections to the primary carriage may be achieved via the secondary carriage. Cable and pipe routings can be a source of disturbance and positioning errors due to the varying forces they exert as the machine axis moves. These conduits may be coupled to the secondary carriage, which has a lower requirement for positioning accuracy than the primary carriage. One or more short, flexible and relatively light connections are then employed to transfer services from the secondary to the primary carriage.

(36) A further embodiment is depicted in FIG. 7. The arrangement of a primary carriage (or support) and a secondary carriage (or driven body) is similar to that shown in FIG. 1. However, in FIG. 7, the carriages are mounted on horizontally extending guideways. In this configuration, the secondary carriage is able to maintain the position of the primary carriage accurately against a constant opposing force. The guideways are mounted on a machine base 40.

(37) A further embodiment of the invention involving a rotary machine axis will now be described with reference to FIGS. 8 to 11. As previously mentioned, there is an angular equivalent to the linear applications described above. This variation is now described using, as an example, a rotary machine axis, with a shaft or spindle as the main guided body, and a secondary rotating element forming the secondary guided body or driven body, with compliant coupling between them. A housing 50 contains a first drive in the form of a precision rotary motor drive arranged to move a support which is constituted by a shaft 52. The shaft 52 is supported on bearings 51 and 53. The drive has a direct drive motor stator 55 attached to the housing 50, and a direct drive motor rotor 57 attached to the shaft 52. Item 54 represents a workpiece, chuck or other tool mounted on one end of the support. The other end of the support is connected via a resilient coupling 58 to a second, relatively low precision drive 60. In the example depicted, this drive includes a worm 59 and gear-wheel 61 drive. As with previous embodiments, it will be appreciated that a range of drive types may be suitable for this purpose having sufficient mechanical advantage, and if possible, internal friction to provide the required non-reversible properties.

(38) Coupling 58 functions as a rotary equivalent of the linear resilient coupling 26. It comprises two parallel, spaced apart discs 62 and 64. A set of eight wedge-shaped members are provided between the two discs at circumferentially spaced locations. Four of these members 66 are mounted on disc 62 whilst the other four members 68 are mounted on the other disc 64. A coil spring 70 is provided between each of the members. Rotation of one disc relative to the other in one direction will compress four of the springs, whilst rotation in the other direction will compress the other four springs. In this embodiment, disc 62 and the associated wedge-shaped members 66 effectively acts as a driven body as described herein coupled to support 52 via resilient coupling arrangement 58.

(39) Coupling 58 is shown as one example of a suitable resilient rotary linkage. Other suitable arrangements will be apparent to the skilled reader.

(40) Embodiments of the invention have been described with reference to carrying a tool or a workpiece on the machine axis, and it will be appreciated that the invention is applicable to use of a machine axis in a broad range of applications requiring precise position control. For example, it may be used in grinding, turning and polishing, and lithographic operations, and inspection of machined components.

(41) FIGS. 12 to 14 relate to embodiments of the present invention in which a counterforce mechanism of the form disclosed herein is employed to provide resilient support for a machine tool whilst maintaining its base in a desired orientation. A machine tool 80 is shown in FIG. 13 which is mounted on four base support units 82. An enlarged view of one of the support units is shown in FIG. 12.

(42) The machine tool includes a machine base or bed 84 with a carriage 86 of significant mass mounted for linear movement along the bed. Movement of the carriage from one end of the bed to the other causes substantial variation in the forces exerted on the support units 82.

(43) Each support unit includes a resilient or compliant coupling, in the form of a spring 88. This provides support for the machine mass whilst allowing relative movement between the machine base and the floor within certain frequency ranges (these frequencies being the range over which the devices are required to isolate). The support units may also include some form of damping, represented by damper units 90 to prevent uncontrolled bouncing of the machine on the springs 88. Each spring is mounted on a height adjustment device 92. Each device includes a motor-driven drive for changing the height of a driven body in the form of a platform 94.

(44) Each support unit is arranged such that as the respective spring 88 is compressed due to a change in vertical force on the unit (for example resulting from movement of the carriage 86), the support unit adjusts the height of platform 94 to alter the counterforce exerted by spring 88 so as to maintain the desired machine base height above the floor.

(45) The height of the machine base above the floor may be monitored using a displacement sensor 96, provided in engagement with the machine base. This generates a signal in response to changes in the height of the machine base in the vicinity of the respective support unit which is fed to a control arrangement of the support unit or machine tool to indicate a requirement for height adjustment. The control of machine height at each support unit position is therefore maintained in a closed-loop manner.

(46) Another machine support implementation is depicted in FIG. 14. The configuration of the support units 82 correspond to that of the units shown in FIGS. 12 and 13. However, the weight of the machine tool is essentially borne by four primary springs 98. The secondary springs 88 associated with the support units are capable of exerting forces on the machine base corresponding to the variation in the load to be supported.

(47) While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, it is not the intention of applicant to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicants' invention.